simd: use fused multiply-add in emulated MulAdd

[ricardoofnl]

The emulated MulAdd (used on architectures other than amd64, arm64,
and wasm) computed xy+z with a plain expression. Whether the gc
compiler contracts xy+z into a hardware FMA is architecture
dependent, so on targets without FMA contraction (for example 386)
the result is double-rounded and disagrees with the arch
implementations.

MulAdd is documented as a fused multiply-add (amd64 VFMADD213PS/PD,
arm64 VFMLA), and the arch conformance tests use math.FMA as the
reference oracle. Use math.FMA in the emulated path so it produces
the fused, single-rounding result on all architectures.

Verified with GOARCH=386 GOEXPERIMENT=simd: the simd test package
passes, and over 800k random float32 triples the updated MulAdd
matches math.FMA on every element, while plain x*y+z diverged on
about 27% of them.

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